The invention relates to corrosion monitoring in general, and more particularly to vapor corrosion monitoring. Corrosion under condensed vapors is a serious problem in steam turbines, pipe lines of natural gas, etc. Acid rains are also of concern. There is a need for vapor corrosion monitoring, e.g., continuous monitoring in situ of the corrosion rate in an environment of potentially corrosive vapor.
Corrosion rate measurements using electrochemical methods have been used for liquid corrosion. These involve electrochemical phenomena between a working electrode and a reference electrode with the assist of an auxiliary electrode. In these measurements the voltage drop (IR) along the potential measuring path must be corrected. The IR voltage drop is a troublesome problem in the use of electrochemical techniques for corrosion monitoring. See for instance: Lindsay F. G. Williams and Russell J. Taylor, "IR Correction: Part I A Computerized Interrupt Method"; "Part II Effect on Corrosion Monitoring", J. Electroanal. Chem. Vol. 108, pp. 290-316 (1980); Florian Mansfeld "Tafel Slopes and Corrosion Rates from Polarization Resistance Measurements" in Corrosion-NACE, Vol. 29, No. 10, October, 1973 pp. 397-402; Monika Berthold and Sigrun Herrmann, "Investigations of Corrosion with Measurement and Compensation of the Ohmic Drop", Corrosion-NACE, Vol. 38, No. 5., May 1982, pp 241-245.
This prior art, however, is concerned with corrosion rates with liquids. The situation is quite different when measuring corrosion rates due to a vapor, for instance for atmospheric corrosion monitoring.
IR voltage compensation in corrosion with vapor becomes a more serious problem in the measurement of electrochemical corrosion rate since there is a large electrolytic resistance between the working and the reference electrode. As a result the IR voltages due to the reference electrode being located in the path of significant current flowing between working and counter electrodes affect the measurement considerably. The IR portion of the voltage measured by the reference electrode is due to an IR voltage drop that appears when current flows in a resistive electrolyte. This resistive electrolyte is due to vapor condensation in a very thin film, thus of very high resistance. For accurate estimates of corrosion rate, this IR contribution must be separated from the true potential change at the working electrode surface. If the IR voltage drops are not compensated for in electrochemical corrosion rate monitoring, the corrosion rate calculation can seriously underestimate the true corrosion rate of the working electrode. The compensation for IR voltage drops is particularly important for linear polarization methods because the small polarization potentials (.+-.10 mV) introduce a high sensitivity to even a few millivolts of uncompensated IR voltage in the corrosion rate calculation.
This IR voltage drop constitutes an especially serious problem in atmospheric corrosion probes since the corrosive environment is a thin condensed film which has high resistance due to its thinness. Therefore, it must be compensated for.
Thus, this IR voltage drop is a major problem limiting the use of the linear polarization corrosion rate measurement in atmospheric corrosion monitoring. Unfortunately, the IR voltage is not only a function of the resistivity of the environment, is also a function of the resistance of the current path. The latter varies with the location of the reference electrode and also depends upon the geometric shape of the corrosive environment. It is known to use a computer in order to ascertain the relation between a particular geometry and the resistance involved in the IR voltage drop. See, for instance paper by John W. Fu entitled "A Finite Element Analysis of Corrosion Cells" In Corrosion, Vol. 38, No. 5, pp. 295-296, May 1982. This paper is hereby incorporated by reference. Due to the complex dependence of IR voltage drop on these parameters, no simple mathematical calculation has been available for IR voltage drop compensation in potential measurements during an electrochemical corrosion rate measurement. To overcome the difficulty, a finite element method for calculating IR voltage drop in corrosion cells has been proposed as explained by John W. Fu in a paper entitled "IR Voltage Correction in Electrochemical Atmospheric Corrosion Probes Using a Finite Element Calculation", presented at the International Corrosion Forum Apr. 6-10, 1981, Toronto, Ontario, Canada. The IR voltage is calculated using numerical solutions of the governing partial differential equations for corrosion cells. It is also proposed in that paper to model the geometric shape of the corrosive environment by an assembly of small elements called an element mesh. The electrode surfaces are modeled by the surfaces of elements at the boundary of an element mesh. The potential at each element location is generated by the numerical solution of the governing differential equations. The difference in potential between the reference electrode surface and the working electrode surface is the IR voltage drop for a set of assumed conditions which include the resistivity of the corrosive environment, the total current flow between the counter and the working electrode and the locations of each of the three electrodes.
Using this method, it has been possible to examine the IR voltage drop as a function of various parameters, thus aiding in the design of the probe that produces the lowest IR voltage drop. Furthermore, for a given corrosion probe design, an IR voltage drop calibration curve (as a function of current flow and resistivity of the corrosive environment) can be generated to compensate the IR voltage drop component in subsequent potential measurements. This indirect and theoretical approach, however, does not provide true geometrical characteristics for an in situ vapor corrosion rate measurement.
On account of the very thin film buildup at the juncture between the corrodent vapor and the exposed surface of the corroding metal, corrosion rate measurements have been proposed to be made in situ within a pipe carrying natural gas with the assist of a planar probe embodying working and reference electrodes, as well as an auxiliary electrode used for the determination of the IR voltage drop. See, for instance, U.S. Pat. No. 4,196,057, and the article by Eddie C. French and Paul B. Eaton entitled "A Flush Mounted Probe for Instantaneous Corrosion Measurements in Gas Transmission Lines", in Materials Performance, pp. 13-18, July 1978.
A general treatment of atmospheric corrosion rates is also to be found in the article entitled "Electrochemical Measurements of Time Wetness and Atmospheric Corrosion Rates" by F. Mansfeld and J. V. Kenkel in Corrosion-NACE, Vol. No. 33, January 1977, pp. 13-16, and in the appendent reference listing.